tf_a1
{ "a1", (void *)offsetof(struct trapframe, tf_a1), db_rw_ddbreg, NULL },
tf->tf_a1 = p->p_psstrp; // ps_strings
tf->tf_a1 = 1;
tf->tf_a1 = (intptr_t)&sf->sf_si;
tf->tf_a1 = retval[1];
tf->tf_a0, tf->tf_a1, tf->tf_pc);